This invention relates to apparatus and methods for the control of pressure in the administration of continuous positive airway pressure (CPAP) treatment or assisted respiration.
The administration of CPAP is common in the treatment of Obstructive Sleep Apnea (OSA) syndrome and Upper Airway Resistance syndrome. It has been postulated that CPAP treatment effectively acts as a pneumatic splint of a patient""s upper airway by providing air or breathable gas at a pressure elevated above atmospheric pressure to the entrance of the patient""s airway. Treatment pressures in the range 4-25 cm H2O are commonly encountered.
Common to all forms of CPAP apparatus is a mask worn by a patient having connection via a flexible air delivery tube to a flow generator. Most often, the flow generator is driven by an electric motor that is under the control of a motor controller. In this specification reference to a xe2x80x9cmaskxe2x80x9d is to be understood as including a nose mask, a mouth mask. a nose and mouth mask in combination, nasal prongs or nasal pillows, or a full face mask.
CPAP treatment can be in a number of forms, including (i) the maintenance of a constant treatment pressure level, (ii) alternating between two constant levels in synchronism with the inspiratory and expiratory phases of respiration (xe2x80x9cbi-level CPAPxe2x80x9d), and (iii) having an autosetting level in accordance with a patient""s therapeutic needs. In all of these cases there must be control over the pressure of air or breathable gas supplied to the patient""s airway.
In one form in the prior art, control over the treatment pressure is achieved by speed control of the electric motor driving the turbine (or fan) that together constitute the flow generator. In the case of bi-level CPAP, the motor must be able to accelerate (or decelerate) respectively to double (or half) its operational speed within about 100 ms. For typical CPAP treatment, this equates to the need to supply (or sink) approximately twice the steady state electrical power within the noted time interval. Disadvantages in motor performance associated with the rapid transitions in speed are, for example, noise due to magnetostrictive effects and bearing vibration, and increased thermal dissipation requirements. Lower noise will increase patient compliance with the treatment.
FIG. 1 shows, as a cross-sectional view, a conventional flow generator 10 comprising a chamber 12 that is segregated from the casing 14 of the CPAP apparatus. The casing 14 houses the control circuitry (not shown) associated with the flow generator 10. The flow generator further is comprised by a motor 16 driving an induced flow centrifugal turbine (impeller) 18, which induces the flow of air or breathable gas by an air inlet 20 to pass the air or breathable gas under pressure by an air outlet 22 to the air delivery tube (not shown) and so to the mask (also not shown). The turbine 18 has radially directed impeller blades 24. The alternate use of axial fans is known also in CPAP apparatus.
Another form of controllable flow generator involves operation of the driving motor at a constant speed, and venting or bleeding-off excess air from the output side of the turbine. As shown in FIG. 2, the turbine 18 is connected to a plenum chamber 30 by a supply pipe 32. The plenum chamber has a controllable spill valve 34 operable to indexingly open and close an opening 36 in the chamber wall to allow the venting of air to atmosphere so as to achieve the desired output pressure at the air outlet 38.
Such an arrangement also has disadvantages. Firstly there is excessive noise due to the venting of air when the treatment pressure is adjusted. This is particularly the case for each expiratory event during bi-level CPAP treatment when the treatment pressure typically is reduced from 16 cm H2O to 6 cm H2O, and thus over one half of the pressure head of the air within the plenum chamber 30 must be vented by the spill valve 34. It is also difficult to maintain precise treatment pressure regulation, since small variations of the spill valve position give relatively large variations in the pressure at the air outlet 38. This configuration also leads to an inherently low maximum flow rate which can compromise the efficacy of CPAP treatment. In particular, the spill valve 34 works by increasing outlet flow from the plenum chamber 30, thereby increasing the pressure drop in the supply pipe 32 and the turbine 18, thus dropping the pressure in the plenum chamber. The combined pneumatic impedance of the supply pipe 32 and the turbine 18 limit the maximum achievable flow rate into the plenum chamber 30, and so to the patient, on subsequent closure of the spill valve 34.
An example of another prior art arrangement that operates on the output of the flow generator can be obtained from International Publication No. WO 90/14121 (PCT/US90/02800), in the name Puritan-Bennett Corp.
As is noted, the invention also has application to apparatus for the provision of assisted respiration. Use of the term xe2x80x9cassisted respirationxe2x80x9d is to be understood as embracing both ventilators and respirators. Ventilators can broadly be characterised as providing for patient ventilation in a volume cycled mode, and do the work of breathing for the patient. Respirators, on the other hand, may or may not do the complete work of breathing for a patient, and are characterised by their bi-level operation, with a large treatment pressure differential between inspiration and expiration and a high inspiratory treatment pressure, which may reach 30-40 cm H2O.
It is an objective of the present invention to overcome or at least ameliorate one or more of the problems associated in the prior art. The gist of the invention is to provide control of output pressure by controlling the efficiency of a flow generator or its component turbine.
Therefore, the invention broadly discloses a controllable flow generator for the supply of breathable gas in the administration of CPAP treatment or assisted respiration, the flow generator comprising: a motor coupled to drive a turbine, an inlet for breathable gas in communication with the turbine, an outlet for the supply of said breathable gas at a pressure elevated above atmospheric pressure, and means to control the efficiency of the flow generator and thus the pressure of breathable gas exiting the flow generator.
In this specification the term xe2x80x9cefficiencyxe2x80x9d in relation to a flow generator or to the component turbine is to be understood as the ability to pressurize a mass of air at a given flow rate and a given pressure.
In one preferred form the control means controls the efficiency of the turbine, and most preferably comprises adjustable pitch turbine blades or turbine louvres. In another preferred form, the control means controls the breathable gas available to the turbine. Alternatively, it controls the gas available to the inlet. Further, the control means can control the impedance of the outlet. The flow generator can further comprise pressure sensor means for sensing the pressure of air or breathable gas exiting the flow generator by the outlet, said sensed pressure being provided to said control means, and said control means further operable to compare said sensed pressure with a set pressure to maintain said exiting pressure substantially the same as said set pressure by controlling the efficiency of the turbine in accordance with the result of said comparison.
The invention further discloses a controllable flow generator for the supply of pressurised breathable gas in the administration of CPAP treatment or assisted respiration, the flow generator comprising a motor coupled to drive a turbine, an inlet for breathable gas in communication with the turbine. an outlet for the supply of said breathable gas at a pressure elevated above atmospheric pressure, and control means for controlling the breathable gas available to the turbine and thus the pressure of breathable gas at the outlet.
Advantageously, the control means acts to restrict the flow generator inlet. The restriction can be over a range of inlet opening. The range can be between the inlet fully open and partly or fully closed. For an operational rotational speed of said turbine, the limits of the range relative to inlet when open and at least partly closed respectively correspond to the highest outlet pressure and the lowest outlet pressure of supplied air or breathable gas.
In a preferred form, the control means can comprise means for closing at least a portion of a mouth of said inlet. Alternatively, said control means can comprise a controllable vane for at least partly restricting said inlet.
Alternatively, the control means acts to adjust the effective entry area of the turbine impeller open to the inlet. The adjustment can be over a range. The range can be between the total effective entry area of the turbine open to the inlet and partial or zero effective entry area of the turbine open to the inlet. For an operational rotational speed of said turbine, the limits of the range relative to said total effective surface area and at last partial effective entry area respectively correspond to the highest outlet pressure and the lowest outlet pressure of supplied air or breathable gas.
In a preferred form, the control means can comprise a positionally adjustable baffle that can block-off at least a portion of the effective mouth area open to the inlet.
In one particular preferred form, the control means acts, to cause the repeating sequential supply of breathable gas at said outlet at a first higher pressure and a second lower pressure. The flow generator can further comprise pressure sensor means for sensing pressure at said outlet, said sensed pressure provided to said control means, and said control means further operable to compare said sensed pressure with a set pressure to maintain said outlet pressure substantially the same as said set pressure by control of the breathable gas available to the turbine in accordance with the result of said comparison.
The invention further discloses CPAP treatment apparatus comprising a patient mask, an air delivery tube connected at one end to the mask, a flow generator connected to the other end of the air delivery tube and comprising a motor coupled to drive a turbine at an operational rotational speed. an inlet to receive breathable gas, an outlet for the supply of said breathable gas to said air delivery tube at a pressure elevated above atmospheric pressure, and control means for controlling the breathable gas available to the turbine and thus the pressure of breathable gas at said outlet.
In one preferred form the control means acts to restrict the flow generator inlet. Alternatively, the control means acts to adjust the effective area of the turbine open to the inlet.
The apparatus can further comprise a path for patient exhalation from said outlet by-passing the turbine. The exhalation path can vent to atmosphere or recirculate to said inlet.
Advantageously, the CPAP apparatus can be for the administration of bi-level CPAP treatment, and the control means is operable between two states respectively corresponding to a desired patient inspiratory treatment pressure, and the reduced desired patient expiratory treatment pressure.
Advantageously, the CPAP apparatus can be for the administration of a treatment pressure adjusting in accordance with patient need, wherein the control means has a continually adjusting current set level corresponding to the desired treatment pressure, and causes the outlet level to be controlled to maintain the pressure at the desired level.
The CPAP apparatus can further comprise pressure sensor means for sensing pressure at said outlet or at a point in said air delivery tube or in said mask, said sensed pressure provided to said control means, and said control means further operable to compare said sensed pressure with a current set pressure to maintain the treatment pressure substantially constant at the desired level in accordance with the result of the comparison.
In another preferred form, the CPAP treatment apparatus can be operable to maintain the treatment pressure at said mask substantially constant by continuous control of said control means.
The invention yet further discloses a method for control of the pressure of breathable gas delivered by a flow generator in the administration of CPAP treatment or assisted respiration, said method comprising the steps of operating said flow generator at an operational rotational speed whereby said breathable gas enters the turbine by an inlet of the flow generator and exits the turbine at a pressure elevated above atmospheric pressure, and controlling the breathable gas available to the turbine and thus the exit pressure.
All the above arrangements limit the through-put of breathable gas through the flow generator, compared with prior art arrangements which provide for pressure control by spilling excess gas, thereby resulting in concomitant increase in acoustic emissions and motor power requirements. Embodiments of the invention as defined can provide a controllable flow generator or CPAP apparatus that provides one or more of the advantages of lower power, lower acoustic noise, higher maximum air flow and improved pressure control in comparison with prior art arrangements. In one or more embodiments, the power supply for the flow generator can halve its rating, thus reducing cost, heat dissipation and occupied volume.
It will be understood that all references to xe2x80x9ctreatment pressurexe2x80x9d include a continuous pressure that can vary with time if desired in accordance with treatment needs, and therefore is not necessarily of a constant level.